


Irreplaceable

by MeGaLoTrash



Category: Undertale (Video Game)
Genre: Codependency, Ecto-Genitalia (Undertale), F/M, Heavy Angst, Hurt/Comfort, Implied/Referenced Suicide, Past Character Death, Past One Sided Fontcest, Referenced Alphyne - Freeform, Suicidal Thoughts, Uncomfortable sex scenes
Language: English
Status: Completed
Published: 2018-02-02
Updated: 2018-02-02
Packaged: 2019-03-12 16:31:17
Rating: Explicit
Warnings: Major Character Death
Chapters: 1
Words: 4,367
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/13551231
Author URL: https://archiveofourown.org/users/MeGaLoTrash/pseuds/MeGaLoTrash
Summary: Of all the timelines that Sans had lived through, he never expected that this would be the one he would end up stuck inSansdyne angst fic for @AzuraNagomii, please mind the tags!





	Irreplaceable

**Author's Note:**

  * For [AzuraNagomii](https://archiveofourown.org/users/AzuraNagomii/gifts).



Of all the timelines that Sans had lived through, he never expected that this would be the one he would end up stuck in. He’d lived through the same series of events far too many times to count; sometimes it would end with the unbridled joy of reaching the surface, but more often than not things went to shit. Sans had seen everyone he loved and cared about die so often that he had grown a little numb to the pain. While he hated himself for coming off so cold, it was necessary if he didn’t want to lose his mind. No matter how often it happened though, the death of his brother always hit him like a freight train. Sans knew the love he had for his brother was immoral, his secret obsession causing him many sleepless nights even before he had to deal with time resetting on top of it all. 

This meant that his brother’s death kept hitting him harder as he fell deeper. In these timelines he would go to drown his sorrows at Grillbys; consuming way more than his usual tolerance just to drown out the voices in his head reminding him that he had failed again. The bartender never asked questions, not while the remaining residents of Snowdin were all in here with their own problems. Grillby kept his glass full, let him pass out at the counter, and left him to wallow in self hatred while he waited for this hell to end.

Only this time, it didn’t end. 

It must have been getting close to a week of him getting blackout drunk before Sans realised that something was up. He had tried to ignore the feeling at first. But when Grillby eased up on giving him drinks he started to panic. The bartender had his best interests at heart, insisting that it was time he started to come to terms with his brother’s death before drinking himself to death, but it had all just been white noise to Sans. He had rushed home and somehow managed to get into his lab even in his drunken stupor, eventually finding his phone in the mess that had accumulated without Papyrus to keep him in check. The date on his screen didn’t lie. Sans could feel his soul sinking fast, he couldn’t remember the last time that it had been this long without a reset. _this couldn’t be it, could it?_ Dread had almost gripped him completely when he felt the phone starting to vibrate in his hand with a vaguely familiar number flashing up on the screen. 

He answered and within seconds he was able to identify the caller as Undyne even through her hysterical crying. She had been trying to get hold of him for days. They hadn’t seen each other since the day of Papyrus’s death but thankfully Undyne understood he needed space. The problem was she hadn’t heard from Alphys in days either, no matter how hard she tried to knock on the lab door. As if Sans didn’t already feel terrible, the realisation as to why Alphys wouldn’t be answering just broke his spirit even more. He’d seen first hand in other timelines just how unstable the royal scientist had been. But he’d never even considered the possibility of her going through with taking her own life considering everything always reset before she had the chance. Not that he blamed her though, if he was honest the thought had crossed his own mind more than once. 

Sans didn’t have the heart to tell Undyne over the phone, especially when there was no logical reason as to why he would know without checking. He made the journey to Hotland with bones that wouldn’t stop shaking, unlocked the lab with his old key and insisted Undyne stay outside while he prepared himself for the inevitable. Even though he knew what waited for him in the dimly lit halls of the lab, actually seeing the pile of dust made his legs almost give way. His reaction was nothing compared to the wail of anguish that came from behind him though. It seemed Undyne had ignored his warnings, and Sans hated himself for feeling relief over not having to deliver the news any more.

He held her while she cried, in what he later realised was probably the first time they had ever touched that intimately. Sans tried to act empathetic, he tried to remember the first time he had seen Papyrus die to understand what she was going through. But he just couldn’t bring himself to snap out of the shell of apathy he’d spent so long building up. Not when he knew just how similar he and Alphys’s solutions would be if he allowed himself to feel again. Still no matter how closed off he was, he could still tell how broken Undyne was. Sans had managed to calm her down enough to take her home, he had even offered to stay with her that night so she didn’t have to be alone. In truth he was being selfish, her grief was a welcome distraction from his own and he was too exhausted to make the trip back himself. 

What had seemed like a good idea in the moment ended up turning into an accidental arrangement. He ended up staying the next night, and then the night after that. Before he knew it he was practically living on the spare mattress on Undyne’s floor. He knew she was too unhinged to be alone and the longer he stayed away from Snowdin the harder it was to think about returning home without Papyrus. So he stayed at her house in Waterfall, waiting for the reset that never came, all while trying to comfort her as best as he could. The thing no one ever tells you about grief is how many emotions are actually involved. Sans saw Undyne go through them all as the days turned into weeks. He wasn’t any better though. He may have seemed more together on the surface, but his unanswered messages on his phone and sleepless nights told another story. 

They were approaching nearly a month together when Undyne’s attitude started to shift. She had started training again, even taking up a morning routine of running to get her fitness back. Grief still clung to her like a shadow but Sans could tell her spark was returning. She made inquiries about who was going to be in charge with Asgore gone and spent the evenings telling him how they could avenge everyone who had fallen. Sans often listened with his sockets closed, concentrating on when her tone raised as she came up with different ideas on how monster-kind could fight back. Her tone and enthusiasm was eerily familiar. If he let himself drift just enough while she was talking it was almost like he could hear his brother being the one saying her words instead. He knew that it wasn’t a healthy way of coping, nor was it fair on Undyne, but it he found himself clinging to every scrap of his brother that he could. 

While it was comforting to see some of her old self returning, Sans knew that he couldn’t sit idly by and let her think that revenge was the best option. He tried to think of ways that he could help, there _had_ to be some way of getting out of this timeline. He had to go back to the lab, to find any and all notes he could about time and the DT experiments. Maybe if he could escape he could track down the human and make them reset by force. He mentioned his idea to Undyne over breakfast one morning, disguising it under trying to find stuff at the lab that would help her instead and she had beamed at him with barely disguised pride. Within minutes he found himself being dragged to her bedroom in order for them to find him something appropriate to wear. He was going to protest, but then he guessed wearing the same clothes for over a month wasn’t exactly acceptable. 

He should have said something though, because there was no way in hell he could refuse when Undyne carefully pulled out an old lab coat from the back of her closet. Sans knew exactly whose coat it was. He slipped it on without protest the moment he saw Undyne’s face light up with hope.

“See! I told you it would fit!” Undyne said triumphantly as she leant over him to brush the front of the coat down. 

They were stood in front of the mirror in her bedroom and Sans found he no longer minded their close proximity. He usually kept everyone at arms length, but his soul felt warmer than it had been in weeks while he was being fussed over. Even if he simultaneously felt the urge to throw up. 

“Hey don’t look so down!” Undyne chided him playfully as she squeezed him into a hug from behind “If it all goes well nerd you can be _my_ royal scientist!" 

The tension in the air was thick even before Undyne had finished her sentence, the significance behind it weighing heavily on both of their minds. But she didn’t move away from the embrace and neither did Sans so they ended up staying frozen in place. 

“i wouldn’t get your hopes up ‘dyne.” Sans finally said just barely above a whisper when his mind kicked into gear and scrambled for something to defuse the situation. “i’m outta practice with the science stuff, i’ll probably get in _alkynes_ of trouble…”

It was weak and he knew it, but Undyne took the bait anyway. 

“Don’t say that! This is something you’re passionate about! I know you’re going to find- wait that was a pun wasn’t it” She finished with a groan while playfully pushing him away, shooing him out of the room so she could get ready for the day. 

Sans might have forced a laugh loud enough for her to hear before he left for the lab, but anything was better than addressing whatever the fuck that was. Still that didn’t stop the lingering feeling of discomfort until he was down in the depths of the true lab. He understood completely why Alphys had tried to keep most of the lab closed off from public knowledge, but it meant any research papers were extremely hard to find. He had become so engrossed with finding a particular study on the DT experiments that he didn’t even know he wasn’t alone until it was too late.

He had seen the ‘failed’ experiments before in other timelines, but it had always been when Alphys had shown them to him after confessing to what had happened. Sans should have known that them being alone for over a month without their caretaker would have changed their docile temperament. But he had been too wrapped up in his head to have ever anticipated he would be running for his life when he stepped into the labs earlier that day. When he did manage to escape he just kept running; ignoring the burning sensation in his bones or the magic pouring freely down his face. 

When Undyne found him he was a shivering mess on her kitchen floor. 

Tea was boiling on the stove by the time Sans came round to some sort of cohesive thought, with the whistle being the catalyst of the start of his breakdown. The shock of situation had caught him off guard, effectively demolishing the state of numbness he’d spent so long building up. Seeing those creatures so full of desperation and despair felt too much like he was looking into a mirror. 

“Sans! Sans- what happened in there?” Undyne’s voice was full of concern as she came rushing over to him, all attempts at making tea forgotten as she searched his face for answers. “You know what it doesn’t matter, you’re safe now. I’m here and you can always try again tomor-”

“no! there’s no point- can’t you see it’s hopeless?” Sans pushed her away, magic prickling in the air around them as several bone attacks formed in the air. He didn’t mean her any harm, but he was so out of control that they had formed on their own. _just another failure to add to the pile._ “we’re so fucked-nothing i do is ever going to make a difference! It’s never going to change anything- i- i can’t bring them back-”

Undyne approached him again cautiously, but she hesitated when more bone attacks appeared in the air.“Sans I-”

“i can’t- i can’t keep going like this! i need to go back or-or-what’s the point?” Sans let out another sob as he felt the hope drain out of his soul completely. Control over his attacks returned to him. Sans turned the attacks inward, towards himself, as a singular thought took over his mind “i’d be better off dead”

“NO!” Undyne screamed and dove towards him with no thought of her own safety. The shock was enough to stop the course of his attacks, the pointed bones hoovering inches above where they were huddled together on the floor. “You can't Sans! I can’t lose another person i care about that way” She pulled Sans face towards hers, wiping the tears away from his skull until he was able to see her anguished face more clearly. “Y-you can’t give up! Pap...Papyrus wouldn't want you too”

Sans could barely hear the sound of his attacks falling to the floor as he gave into his grief and allowed Undyne to pull him into a tight embrace. She was completely right; not only could he not put her through that in his right mind, he knew that Papyrus would want him to keep going no matter what. It was so hard to come to terms to living when Papyrus had left such a void in his, but Sans had how killing himself would work across timelines. There was no way he could risk not coming back with a reset and never seeing Papyrus again. He sobbed harder, clutching at Undyne’s shoulders as he began to comprehend that this timeline might be his lot. 

“i can't i-i” Sans spoke through his sobs, ignoring Undyne’s attempts at soothing him. He knew his mouth was running before his mind, but he no longer cared if he slipped up and said something stupid. He just couldn’t keep it all in any more. “i-i miss him so much, i just need him- i want- i want to feel-i can’t!”

Undyne cut off another one of his sobs by bringing his face into her hands again. The look she had on he face was hard to read, and Sans realised he’d probably messed up the one decent relationship he had left with his half confession. Which was why when she lent in to place a kiss on his teeth his his eye lights all but disappeared. Sans could feel part of his mind screaming at him and asking him what the fuck he was doing, but Undyne wasn’t pulling away. He could see her eye screwed shut as her hands pulled him closer by his jacket, like kissing him was the only thing that mattered to her. Sans heard a whimper that sounded like it came from him as he opened his mouth and let his sockets slip shut. 

It took a bit of coaxing on Undyne's part but his tounge did eventually form. The moment he felt it press against hers he couldn’t hide the moan that shook his whole body. It felt good. _really good._ Even though logically he knew this wasn’t the right solution to his problems, it was hard for him to think clearly when his body was responding so willingly. Undyne was so close, and she was drawing his smaller frame practically into her lap as she deepened the kiss. Sans knew that what they were doing wasn’t the best idea, but it was what he needed to feel alive again. 

Sans knew he was getting carried away already. His hands had at some point moved to hold Undyne back but it was only when he tangled one into her hair did he realise what he was doing. He was kissing a friend, a friend who was still in love with someone else. A friend who wasn’t Papyrus. Tears started to stream down his face as his soul ached for another’s touch. But Papyrus was gone, dead, and even if he was alive there was no way he would ever want to do this. Undyne had to draw back from him then as his sobs became harder to ignore. But she didn’t look annoyed or offended that he was still sobbing uncontrollably even after the kiss.

She looked like she understood. 

“i’m sorry-!” Sans tried to babble an apology as he struggled to breath “it’s not you- i can’t- oh gods i can’t stop crying- i just-”

Undyne silenced him with a hand over his mouth. She patiently waiting for his breathing to regulate, which took a lot less time than he anticipated. There was just something about the way she was controlling his movements that was soothing to his soul. The way she brought him close so that their foreheads were touching was so familiar that he couldn’t help melting into her further.

“It doesn’t matter Sans, it’s ok. I’m here” 

_how could he not believe her?_

Sans was the one the initiate the kiss this time, pushing himself flush against her so that every possible part of their bodies were touching. He was desperate to feel again, he was even more desperate to forget everything else. Even if it was temporary, it was the relief he needed. It was what they _both_ needed. Undyne was more than eager to take the lead again, gripping him tightly as she deepened the kiss. With them pressed so close together every movement felt like it was igniting a fire in his bones. It became clear quickly with the way she was grinding up into him that a kiss wouldn’t be enough. 

To her credit Undyne built them up slower than he expected, even though every touch made him instantly want more. She took everything in her stride, only pausing momentarily when she pulled down his pants. Undyne regarded the soft blue glow of his cock with a kind of puzzled wonder as Sans buried his skull in the crook of her neck to hide his flush of shame. The first touch had him gasping for air; it had been so long since he had even formed anything, and so much longer since he had been with anyone. Her hands were warm and rough, the hands of someone who worked hard and fought for what they believed in. If it wasn’t for the texture of her scales it would have been easy to mistake the touch for someone else's. 

But Undyne was the one touching him. They was no point in pretending that he wasn’t currently a whimpering mess on her kitchen floor. He would just have to try to suppress that lingering feeling of guilt until they were done. 

Undyne shifted their positions so that Sans was on his back before making quick work of stripping off her own clothing. Sans found himself staring; he’d always thought that Undyne was reasonably attractive and the sight underneath her clothing made his magic start to throb with anticipation. He clung to that feeling above everything else, focusing on the heat of her body returning when she climbed on top of him. He needed this, it may not have been right but it couldn’t be completely wrong when it was making him feel better than he had in weeks. 

“Is this ok? I mean- do you want me to continue?” Undyne’s voice was strained even if it still held the same commanding tone she had earlier. She wasn’t unsure but she was letting Sans decide if this was going to go any further. 

Instead of an answer he nudged his hips upwards until the head of his cock was pressing at her slick folds. Thankfully that was all the answer she needed. Undyne’s pace was surprisingly slow at first but as soon as Sans starting moving with her that changed. Undyne had prompted him to sit up as they moved together so they could continue kissing. He could feel their mixed sweat dripping down onto his borrowed coat as their movements became uncoordinated. Undyne was moaning into the kiss now too, and Sans could feel the familiar stirrings of climax approaching. His cock was being gripped by the soft inner walls of her pussy as he pulled back to warn her. Even through his hazy mind he knew cumming without warning would be something he would regret. 

“fuck i’m- i’m ah” Sans stumbled over his words. Gods, he was bottoming out inside her and he still couldn’t bring himself to actually say it. 

“It’s ok, i’m close too. Cum for me S-Sans”

Undyne might have hesitated on his name, but Sans did as he was told. He grabbed hold of her hip with one hand to give himself something to thrust against and used his other hand to awkwardly stimulate her clit. Somehow they managed to get off together with each others moans managing to drown out any of their own embarrassment. Sans felt numb as soon as he came down, but he still went along with more kisses and let Undyne lead him into the bathroom to clean up. They fucked again that night, in her bed this time, and it was only slightly less awkward. 

Just like he had accidentally moved into her house, they fell into a sort-of relationship that was built on comfort alone. Every night he convinced himself that caring for her was enough, and every morning he woke up wishing that it was all a dream. Undyne might have seemed like she had everything going for her with the way she was fast becoming the only contender to the crown, but Sans knew differently. He held her when she broke down, kissed away her tears, and wilfully ignored when she whispered a name that wasn’t his own. He could hardly judge when he’d let slip more times then he could count. 

As Undyne rose to power the underground started to talk. Mostly about the policies she planned to install, but Sans didn’t miss the way monsters whispered about him too. He knew it was obvious what was going on between them, but the whispers about how he was trying to replace his brother still stung. He knew Undyne had similar rumours being spread, but he couldn’t bring himself to say anything in response. Sans went along with everything, staying right by her side as everything fell into place. He still didn’t feel right about what she wanted to do as a ruler, but he guessed that supporting her was the right thing to do in his position. Monsters in a relationship were meant to support each other. 

The first night at the palace Undyne insisted that he stay with him, not caring about how much it was going to fuel her enemies. Since their first time she had only grown in confidence, easily coaxing his magic into forming a slick entrance. When he let slip that he could change his magic Undyne had been more than eager to pleasure him that way. While Sans had been reluctant at first, he couldn’t deny that he found it easier to drown everything out with her head between his legs. This way he could let his fantasies run away with him.

“oh shit-p- ah oh- fuck! undyne!” Sans screamed as he came, his eye sockets screwed shut as he tried his hardest not to fuck up. Not tonight, not when it meant so much to her. 

Usually he’d hear some kind of retort, a chuckle or he would feel Undyne increasing her efforts. But there was silence, in fact he couldn’t even feel her fingers inside him any more. 

His sockets snapped open to a familiar sight, but it took him a moment to realise he was actually in his old bedroom. In his house. With the distinct sound of Papyrus in the kitchen downstairs. Sans wept with joy, only stopping to throw himself at his brother when he came to check if Sans was OK. Papyrus found his tears bemusing but thankfully didn’t deny him the hug he’d been craving for months Sans collapsed into his brothers arms, the smile never fading from his face. 

It took Sans a little time to remember his old routine, but Papyrus’s enthusiasm was hard not to fall in line with. While his mind and body begged him for rest he dragged himself along to Papyrus’s royal guard training. After so much time apart, Sans couldn’t bear to let him out of his sight. And a small part of him really wanted to see how Undyne was, no matter how hard his soul hammered in his chest when she opened her front door.

Thankfully Papyrus took up most of her attention, eagerly throwing himself into the cooking lesson while Sans tried not to fall asleep at the table. It was only as they were leaving when something felt amis. Papyrus was already out the door but Sans turned back to thank her for entertaining him for the evening as well. 

“hey undyne i-” Sans’s sentence died on his tongue. For a split second he saw that familiar look on her face, the one he had started to grow fond after seeing it so many times next to him as he fell asleep. _did she remember?_ “i wanted to-heh. i mean thanks for...everything”

Undyne smiled then; a full smile that took up her whole face. One that made him realise just how little of herself he had actually seen in all their time together. She brought him into a tight hug, leaning in to whisper just before letting go. 

“Anytime Sans, you know i’m always going to be there for you.”


End file.
